There has been considerable interest in recent years in the development of a monitoring system for obtaining a continuous measurement of a patient's blood pressure. One of the most promising techniques for obtaining such a continuous measurement involves the use of an arterial tonometer comprising an array of small pressure sensing elements fabricated in a silicon "chip." The use of such an array of sensor elements for blood pressure measurements is disclosed generally in the following U.S. Patents: U.S. Pat. No. 3,123,068 to R.P. Bigliano, 3,219,035 to G. L. Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick, 4,269,193 to Eckerle, and 4,423,738 to P.M. Newgard, and in an article by G. L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med. Elec., April 1963, pp. 73-81).
In a typical tonometric technique for monitoring blood pressure, a transducer which includes an array of pressure sensitive elements is positioned over a superficial artery, and a hold-down force is applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured, and the transducer is positioned such that more than one of the individual pressure-sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. The element that is substantially centered over the artery has a signal output that provides an accurate measure of intraarterial blood pressure. However, for the other transducer elements, the signal outputs generally do not provide as accurate a measure of intraarterial blood pressure as the output from the centered element. Generally, the offset upon which systolic and diastolic pressures depend will not be measured accurately using transducer elements that are not centered over the artery. In some prior art arrangements the pressure sensitive element having the maximum pulse amplitude output is selected, and in other arragnements the elements having a local minimum of diastolic or systolic pressure which element is within substantially one artery diameter of the element which generates the waveform of maximum pulse amplitude is selected. The latter method is shown in the above-mentioned J. S. Eckerle Patent No. 4,269,193. the selection method disclosed in Patent No. 4,269,193 generally identifies the correct transducer element to be used. However, pressure readings provided by individual elements of a transducer array may not be perfectly accurate due to any number of factors. Even small errors in the pressure reading may result in the selection of an incorrect transducer element using the system disclosed in Patent No. 4,269,193, in which case the blodd pressure measurements are inaccurate. A method for determining the correct transducer element for measuring blood pressure is disclosed in copending application Ser. No. 927,843 assigned to SRI International. The method disclosed in present invention represents an improvement on the method disclosed in the above-referenced patent application.